<template>
  <div class='bc-starts'>
    <div class="header" id="demo">
      <div class="top_logo"></div>
      <div class="canvaszz"> </div>
      <canvas id="canvas"></canvas>
    </div>
  </div>
</template>
<script>
export default {
  components: {},
  name: "",
  data() {
    return {};
  },
  methods: {},
  created() {},
  mounted() {
    var canvas = document.getElementById("canvas"),
      ctx = canvas.getContext("2d"),
      w = (canvas.width = window.innerWidth-200),
      h = (canvas.height = window.innerHeight-180),
      hue = 217,
      stars = [],
      count = 0,
      maxStars = 1300; //星星数量

    var canvas2 = document.createElement("canvas"),
      ctx2 = canvas2.getContext("2d");
    canvas2.width = 100;
    canvas2.height = 100;
    var half = canvas2.width / 2,
      gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
    gradient2.addColorStop(0.025, "#CCC");
    gradient2.addColorStop(0.1, "hsl(" + hue + ", 61%, 33%)");
    gradient2.addColorStop(0.25, "hsl(" + hue + ", 64%, 6%)");
    gradient2.addColorStop(1, "transparent");

    ctx2.fillStyle = gradient2;
    ctx2.beginPath();
    ctx2.arc(half, half, half, 0, Math.PI * 2);
    ctx2.fill();

    // End cache

    function random(min, max) {
      if (arguments.length < 2) {
        max = min;
        min = 0;
      }

      if (min > max) {
        var hold = max;
        max = min;
        min = hold;
      }

      return Math.floor(Math.random() * (max - min + 1)) + min;
    }

    function maxOrbit(x, y) {
      var max = Math.max(x, y),
        diameter = Math.round(Math.sqrt(max * max + max * max));
      return diameter / 2;
      //星星移动范围，值越大范围越小，
    }

    var Star = function() {
      this.orbitRadius = random(maxOrbit(w, h));
      this.radius = random(60, this.orbitRadius) / 8;
      //星星大小
      this.orbitX = w / 2;
      this.orbitY = h / 2;
      this.timePassed = random(0, maxStars);
      this.speed = random(this.orbitRadius) / 50000;
      //星星移动速度
      this.alpha = random(2, 10) / 20;

      count++;
      stars[count] = this;
    };

    Star.prototype.draw = function() {
      var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
        y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
        twinkle = random(10);

      if (twinkle === 1 && this.alpha > 0) {
        this.alpha -= 0.05;
      } else if (twinkle === 2 && this.alpha < 1) {
        this.alpha += 0.05;
      }

      ctx.globalAlpha = this.alpha;
      ctx.drawImage(
        canvas2,
        x - this.radius / 2,
        y - this.radius / 2,
        this.radius,
        this.radius
      );
      this.timePassed += this.speed;
    };

    for (var i = 0; i < maxStars; i++) {
      new Star();
    }

    function animation() {
      ctx.globalCompositeOperation = "source-over";
      ctx.globalAlpha = 0.5; //尾巴
      ctx.fillStyle = "hsla(" + hue + ", 64%, 6%, 2)";
      ctx.fillRect(0, 0, w, h);

      ctx.globalCompositeOperation = "lighter";
      for (var i = 1, l = stars.length; i < l; i++) {
        stars[i].draw();
      }

      window.requestAnimationFrame(animation);
    }

    animation();
  }
};
</script>
<style lang="scss" scoped>
.bc-starts {
  width: 100%;
  height: 100%;
  flex-grow: 1;
  // overflow: hidden;

  .header{
    width: 100%;
    height: 100%;
  }
  #canvas{
    width: 100%;
    height: 100%;
  }
}
</style>